The invention concerns an apparatus and a method for an optical measurement of an object to be measured. The apparatus and the method are particularly suitable for measuring tasks in the microsystem technology.
For measuring areas of height structures also on stepped objects optically with highest precision, the vertically scanning white-light interferometry (WLI) is particularly suitable. Common interferometric arrangements for this purpose are the Michelson interferometer, the Linnik-interferometer and the Mirau interferometer. In the WLI, a white light source, typically a halogen lamp is used for illumination. During the measurement the optical travel distance difference between the measuring beam path and the reference beam path is continuously increased or reduced while at a distance of less than 100 nm (nanometers) interference images of the object are recorded generally by a pixel sensor with areal resolution (for example a CCD- or CMOS array). The optical travel distance change can be generated by a movement of the object being measured in the direction of the interferometer, a movement of the interferometer in the direction toward the object or away therefrom, a movement of the interference objective or a reference mirror. This procedure is called vertical scanning A. The intensity curve for each camera pixel, the so-called correlogram which depends on the optical travel length difference is then submitted to further signal evaluation.
In the WLI signal evaluation a distinction is made between a coherence peak evaluation which provides for a relatively coarse estimation of the height location of a measuring point with deviations of at times, more than 100 nm, and the phase evaluation, which provides for measurement errors in the nano or sub-nano range. The height measurement range can be several millimeters.
Lateral geometric features of objects to be measured can be determined via a digital evaluation of pixel images, for example, using an edge detection algorithm. In connection with microscopic objects, measuring microscopes are consequently equipped with suitable cameras in order to record such pixel images and subsequently evaluate them digitally. An advantage of this procedure resides in the high measuring speed which, with a corresponding synchronization between the image establishment and the excitation of the measuring object, permits also examinations of the dynamic behavior of the respective measuring objects to be performed. However, all microscopic procedures are, with regard to the achievable lateral resolution, subject to the limitations set by the diffraction-limited mapping. With the use of visible light, this results generally in lateral resolutions of about 0.5 μm (micrometer).
If the evaluation of lateral structures based on the digitized light-microscopic imaging is to be performed with one of the above-mentioned interferometer arrangements, the interference effects necessarily occurring in connection with the interference microscopy because of the additional image contrasts caused thereby are troublesome.
Therefore, WO 2005/108915 A1 proposed a measuring arrangement which can operate with the same objective in an interferometric as well as in an imaging operating mode. To this end, two light sources are provided which emit light of different spectral composition. In this reference light path a filter is arranged which permits only light of the one spectral composition to pass but not light of the other composition.
With this principle the use of different types of light is provided in the different modes of operation.
It is the object of the present invention to improve this situation.